1. Field of the Invention
The present invention relates to an optical scanning apparatus and a method for adjusting the optical scanning apparatus. The optical scanning apparatus is suitable for use in an image-forming apparatus, such as a laser beam printer (LBP), a digital copy machine, and a multifunction printer, which performs electrophotography processes.
2. Description of the Related Art
In general, an optical scanning apparatus, such as a laser beam printer (LBP), optically modulates a light beam emitted from a light source in accordance with an image signal.
The optically modulated light beam is periodically deflected by a light deflector including, for example, a polygonal mirror, and is focused by an imaging optical unit having fθ characteristics on a surface of a photosensitive recording medium so that a spot image is formed on the surface.
FIG. 13 is a schematic diagram illustrating the main part of a known optical scanning apparatus.
Referring to FIG. 13, a light beam emitted from a light source 1001 is collimated by a collimator lens 1002, and an aperture stop 1003 limits the light beam that passes through. The light beam that passes through the aperture stop 1003 is incident on a cylindrical lens 1004.
The collimated light beam that is incident on the cylindrical lens 1004 is output without change in the main-scanning cross section. In the sub-scanning cross section, the light beam converges so that a substantially linear image is formed on a deflecting surface (reflective surface) 1095a of a light deflector 1095 including a polygonal mirror.
The light beam deflected by a deflecting surface of the deflecting unit 1095 is guided to a surface to be scanned 1008 through an imaging optical unit (imaging optical system) 1006 having fθ characteristics.
The deflecting unit 1095 is rotated in the direction shown by the arrow A, thereby causing the light beam to scan the surface to be scanned 1008 in the direction shown by the arrow B.
Recently, the size of image forming apparatuses has been reduced and the definition thereof has been increased. Accordingly, there has been a demand for optical scanning apparatuses having small, high-definition optical systems. In particular, there is a strong demand for a small polygonal mirror, which functions as a deflecting unit, and a drive motor for rotating the polygonal mirror.
To satisfy such a demand, various kinds of optical scanning apparatuses have been suggested which include, as a deflecting unit, a resonant deflecting element having a deflecting surface that reciprocates around an axis (see Japanese Patent Laid-Open No. 2002-182147).
FIG. 14 is a schematic diagram illustrating the main part of a typical resonant deflecting element.
Referring to FIG. 14, a resonant deflecting element 1056 includes a frame body 1058 and a deflector 1057 positioned inside the frame body 1058. The deflector 1057 is held by two torsion bars 1059 extending in a sub-scanning direction that is perpendicular to a main scanning surface (deflection scanning surface).
Attractive and repulsive forces generated by electrostatic force, magnetic force, Lorentz force, etc., are applied between a deflecting surface 1057a and the frame body 1058 so that the deflecting surface 1057a can reciprocate.
A light beam from a light source (not shown) is incident on the deflecting surface 1057a of the deflector 1057, and is deflected in response to the reciprocating motion of the deflecting surface 1057a. 
The speed of the reciprocating motion can be increased by using the resonant deflecting element under a resonant condition of sine vibration. Recently, the speed has been increased to a level that cannot be achieved by a rotary deflecting element using a polygonal mirror.
In addition, the size of the resonant deflecting element has been reduced due to micromachining technology involving semiconductor processes. The power consumption performance, noise reduction performance, etc., of the resonant deflecting element are also superior to those of the rotary deflecting element.
There are several problems in using the resonant deflecting element. One of the problems is the size of the deflecting surface of the resonant deflecting element.
As mentioned in Japanese Patent Laid-Open No. 2002-182147, the resonant deflecting element has a problem that it is difficult to increase the size of the deflecting surface. Even though it is possible to increase the size of the deflecting surface, there is a trade off between the increase in the size of the deflecting surface and the performance, such as speed, deflecting angle, jitter suppression, etc., of the optical deflecting unit.
The size of the deflecting surface is a factor that determines the laser-beam spot diameter on the surface to be scanned. If the size of the deflecting surface is small, the spot size on the surface to be scanned is large. In such a case, high-precision printing cannot be performed.
According to Japanese Patent Laid-Open No. 2002-182147, the above-described problem is solved by forming a so-called over-field scanner (OFS) optical system that causes a wide light beam to be incident on the deflecting surface.
The second problem is surface accuracy (warping) of the deflecting surface. The deflecting surface of the resonant deflecting element is not formed of a metal block as in a polygonal mirror, but is formed of a thin body with a thickness of several hundreds of micrometers that is made of a semiconductor material such as silicon (Si). Thus, the deflecting surface can be effectively driven with low power consumption.
Since the thickness of the body that defines the deflecting surface is small, the surface accuracy of the deflecting surface is lower than that of the deflecting surface of the polygonal mirror.
In addition, a reflectance-increasing film and a protecting film are deposited on the silicon body. Therefore, tension is applied by these films, which causes the deflecting surface to warp in the form of a second order function.
When the deflecting surface is warped, the light beam cannot be focused on the surface to be scanned and the image quality will be degraded.